Well, is the stacking a unit ability or an item ability? I'm not sure if this would work for items, but for units you would have a trigger something like
"A unit learns an ability" for your event then have a condition "ability learned equal to (insert name of ability, like devotion aura or something) and then if you need multiple levels go into integer and set level of (ability just learned) = 1, 2, 3, w/e. This way you can, depending on the level learned, have multiple auras stacking and add more auras if, say, your hero is leveling up this ability.
For the action part of the trigger, just have the unit learn an ability that you want to stack. If this is for a hero, make sure the ability you add is not a hero ability or it might take up some of your skill points. Anyway, here is the trigger I would use:
Untitled Trigger 001
Events
Unit - A unit Learns a skill
Conditions
(Learned Hero Skill) Equal to Human Paladin - Devotion Aura
Actions
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Level of Devotion Aura for (Triggering unit)) Equal to 1
Then - Actions
Unit - Add Elune's Grace to (Triggering unit)
Else - Actions
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Level of Devotion Aura for (Triggering unit)) Equal to 2
Then - Actions
Else - Actions
Do nothing
Anyway, this is how I'd do it for a hero or another unit. I'd bet you could do something similar for an item, but I haven't tried it yet.
Hope that helps somewhat.